linux-kernel - Re: [RFC] Removing b_end

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Message-ID: <20201025155652.GB5691@mit.edu>
Date:   Sun, 25 Oct 2020 11:56:52 -0400
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-block@...r.kernel.org, Christoph Hellwig <hch@....de>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [RFC] Removing b_end_io

On Sun, Oct 25, 2020 at 04:44:38AM +0000, Matthew Wilcox wrote:
> @@ -3068,6 +3069,12 @@ static int submit_bh_wbc(int op, int op_flags, struct buffer_head *bh,
>  	}
>  
>  	submit_bio(bio);
> +}
> +
> +static int submit_bh_wbc(int op, int op_flags, struct buffer_head *bh,
> +			 enum rw_hint write_hint, struct writeback_control *wbc)
> +{
> +	__bh_submit(bh, op | op_flags, write_hint, wbc, end_bio_bh_io_sync);
>  	return 0;
>  }
>

I believe this will break use cases where the file system sets
bh->b_end_io and then calls submit_bh(), which then calls
submit_bh_wbc().  That's because with this change, calls to
submit_bh_wbc() --- include submit_bh() --- ignores bh->b_end_io and
results in end_bio_bh_io_sync getting used.

Filesystems that do this includes fs/ntfs, fs/resiserfs.

In this case, that can probably be fixed by changing submit_bh() to
pass in bh->b_end_io, or switching those users to use the new
bh_submit() function to prevent these breakages.

						- Ted